The present invention relates to an apparatus and method for measuring power and more particularly to an apparatus and method for measuring variable frequency power in a three phase AC machine.
Adjustable speed drives have been in use for several years, and they are widely used for controlling the speed of induction motors. Such adjustable speed drives offer several advantages over fixed speed drives. For example, adjustable speed drives will increase the productivity of industrial machines since the machine speed can be selected for maximum output consistent with good product quality. Adjustable speed drives will also make industrial machines more flexible so that when a product change requires a different drive speed, the new speed is easily selected. This benefit eliminates the need for gear or belt ratio changes. Furthermore, electronic adjustable speed drive systems together with process or programmable controllers enable the controlling of machine speed, fan speed or pump speed thereby further increasing the productivity and versatility of any machine utilizing such adjustable frequency drives.
In general, motor speed can be controlled by varying factors such as line frequency, the number of motor poles, and motor slip. By controlling the motor speed by varying the frequency, a continuously variable, highly efficient control throughout the entire speed range may be achieved. Furthermore, such a control system is applicable to widely used, three-phase squirrel-cage motors.
Variable frequency may be provided to input terminals of an AC motor in the following manner. A main three-phase supply is first rectified and smoothed in a rectifier or converter section. This DC power is then fed into an inverter section, the current output of which is a sine wave of variable frequency and amplitude. To control the speed and torque of a three-phase, squirrel-cage motor it is necessary to modify the output voltage in proportion to the output frequency to obtain the required torque at a specific speed.
Two different methods of obtaining the variable frequency output may be used. In a six step system (PAM), the DC voltage obtained in the converter is varied. In a Pulse Width Modulation system (PWM), the frequency and the voltage are controlled by varying the pulse width within the inverter. The current output of the PWM system is a sine wave and has far superior form to output waveforms produced by the PAM system. For both types of systems, the goal is to generate a current waveform that approaches sinusoidal with the harmonic components of the waveform at a minimum to ensure minimized torque pulsation and temperature rise. In neither case, however, has this goal been completely met by known devices.
In many situations, the effectiveness of a drive can be further increased by accurately measuring the power output. Measuring the instantaneous power input to a machine or process provides a great deal of valuable information. This measurement which can be used as a feedback signal may be utilized to: automatically adjust the machine feedrate; signal the beginning or end of a process; detect malfunctions or problems; and, indicate, without contact, the flowrate, viscosity or pressure.
Load controls that sense power, have set points and analog outputs are widely used in machine tools, chemical processes and material handling. Unfortunately, such load controls do not work on variable frequency sources. In order for a variable frequency power sensor to be of practical use as a machine controller, the sensor must have the ability to: accurately measure power at both low and high frequencies; provide a fast response with low ripple and immunity to noise; and, have the capability of working on both pulse amplitude modulation drives and pulse width modulation drives.
The main use of variable speed drives is to power induction motors. To measure this power, the lag of the current behind the voltage (or power factor) must be considered. Traditional watt sensors rely on sensing the zero crossing of the sinusoidal voltage and current for power factor calculation. Typical waveforms from PWM and PAM drives show that the waveform is not clean enough for precise zero crossing measurement. A secondary problem is the measurement of current. Many sensors use a current sensing torroid or a lamination transformer, but these devices are not reliable at low frequencies. The combination of zero crossing and current measuring difficulties means that typical watt transducers do not work on the output of a variable frequency drive.
The measurement of AC power requires the multiplication of voltage, current and a power factor so that the equation is: EQU P=V.times.I.times.Cos .phi.
A simple and reliable method for performing this computation electronically is by means of a Hall generator. A Hall generator is a magneto-sensitive semiconductor which, when driven by an electric current and exposed to a magnetic field, generates a voltage that is proportional to the product of current and field. To utilize a Hall generator to measure power, a Hall device excitation current I is derived from a line voltage, and the phase load current produces a proportional field B in the magnetic circuit. The Hall generator exposed to this field generates an output voltage proportional to the product of I, B, and the phase angle between them. The output contains an AC component and a DC component. The AC component can be filtered out if necessary, and the expression for the DC component is V=k.times.I.times.B=k.times.V.times.Icos .phi.=k.times.P where k is a constant representing the Hall voltage. The DC output voltage is therefore a measure of the AC power. With such traditional power measurements as described above, only one or two phases are measured, and as a result, there is a large ripple component in the resulting output.
In three-phase power measuring devices which are used for fixed frequency power sensors, either one or two transducers are utilized to measure either one phase under the assumption that the load is balanced or two phases, respectively. A computer simulation of either of these approaches at various power factors shows that the output would have a large ripple component which is unacceptable for control operations.
For many control applications, fast response is also critical. Typical response time for watt transducers is 250 to 500 milliseconds. The slow response is due in great part to filtering circuits. For a power sensor to be useful, the response times should be reduced to about 15 milliseconds.
A power transducer must also live in close proximity to the variable frequency drive, and such drives generate a great deal of RF noise from the high frequency switching. Therefore, both the housing of the sensor and the internal circuitry should be designed to minimize RF noise.
It is therefore a principal object of the present invention to provide an apparatus and method for sensing variable frequency power that is accurate, reliable and which provides an output signal that does not exhibit a large ripple component.
It is another object of the present invention to provide an apparatus and method for measuring variable frequency power which will be sensitive at both low and high frequencies and will provide a fast response time.
A further object of the present invention is to provide an apparatus and method for measuring variable frequency power which will provide independent and precise machine control and protection.
A still further object of the present invention is to provide an apparatus and method for sensing variable frequency power which will provide a linear output and which is extremely forgiving to gross overloads.